PCC（Pyrindium Chlorochromate）氧化
PCC可以将伯醇和仲醇氧化成醛和酮。由于在有机溶剂中反应，一般不会将醇氧化到羧酸。但当反应体系中有水时，生成的醛酮会形成水合醛或水合酮，进而继续氧化得到羧酸。因此反应体系中要求无水。
PCC中所用的碱除吡啶外，也可以是其它碱，且随着碱性部分碱性的增强，氧化的选择性也提高。其中，DMAP·HCrO3Cl为适用于烯丙醇类及苄醇类的选择性氧化试剂。
PCC的氧化以均相反应为主，但有的方法是将催化剂吸附于硅胶、氧化铝等无机载体或离子交换树脂等有机高分子载体上，对醇作非均相催化氧化。后处理简单并可控制反应的选择性。
将三氧化铬（1eq）加入到6M盐酸（1.1eq）中得到氯铬酸溶液，0℃下加入吡啶（1eq）得到橙黄色的PCC固体，易于储存和操作。在二氯甲烷的悬浮溶液中可以将醇氧化为醛酮。另外3Å分子筛，HOAc，PPTS，PTFA，Ac2O可以加快PCC的氧化速度。可以用亚硫酸钠快速淬灭反应
氧化的一般操作步骤：
Approximately, 1.1–7 equiv., typically1.5 equiv. of solid PCC are added over a ca. 0.01–0.25 M solution of thestarting alcohol in dry DCM. The resulting mixture is stirred at roomtemperature. Very often, ca. 0.2–1.2 g of activated MS per mmol of alcohol areadded in order to accelerate the reaction. In order to moderate the acidity ofPCC, it is very common to add ca. 0.3–1 equivalents of NaOAc. A solid support,such as silica gel, Celite, Florisil or magnesium sulfate, is added, very oftenin a proportion of ca. 0.3–2 g of solid support per mmol of alcohol, in orderto facilitate the work-up. Occasionally alumina, working both as a solidsupport-used to facilitate the work-up and as an accelerant, mixed with PCC isadded, in a proportion of ca. 0.4–1.5 g of aluminaper mmol of alcohol. Normally, PCC is deposited over the alumina. Occasionally,ca. 10–20 equivalents of acetic acid are added in order to accelerate thereaction.
Sometimes, the reaction flask issonicated with ultrasound in order to fragment the surface of the PCC particlesand, therefore, accelerate the reaction.
Although in PCC oxidations, it is verycommon to add simultaneously to the reaction an accelerant, a buffer and awork-up-facilitator; it is not common to employ simultaneously two materialsbelonging to the same kind, with the exception of the combination of the twoaccelerants molecular sieve and acetic acid, which are very often usedtogether.
Work-up:
When a TLC analysis shows that most ofthe starting alcohol is consumed, the solids suspended in the reaction andthe chromium species are removed by filtration through a pad of Florisil1,silica gel, alumina or Celite, and the pad is washed with an organic solvent,such as ether, DCM, or EtOAc. Sometimes, the solids can be removed bydecantation. Other times, it is advisable to add some diethyl ether to thereaction mixture before the filtration, in order to promote the separation ofreduced chromium species in a granular form. Occasionally, the reaction mixtureis concentrated before the addition of diethyl ether. Finally, the collectedorganic phases are concentrated at the rotary evaporator, giving a crudealdehyde or ketone that may need some further purification.
A 500-mL, round-bottomed flask equipped with a 4.5-cm, egg-shaped Teflon-coated magnetic stir bar ischarged with 130 mL of CH2Cl2, the alcohol prepared in Step A (10.4 g, 40.0 mmol), and 15 g of freshly powdered 3 Å molecularsieves. Pyridinium chlorochromate (21.5 g, 100mmol) is added portionwise over 10 min and the resulting mixture isstirred at room temperature for 15 hr. Ether (200 mL) is added slowly with vigorous stirring and thesolution is filtered under vacuum through a pad of 35 g of Celite. The solids remaining in the reactionflask are transferred to the Celite pad by scraping with a spatula and washing with three 50-mLportions of ether. The resulting cloudybrown filtrate is concentrated by rotary evaporation at room temperature togive a brown solid. To this solid is added 25 mL of 1:1 ether:hexaneand the solids are scraped with a spatula. The mixture is then poured onto 60 g of Whatman 60 Å (230-400 mesh) silicagel packed in a 4-cm diameter chromatography column and the liquid is adsorbedonto the silica gel by gravity. The material remaining in the flask is furtherwashed with 1:1 ether:hexane and transferred ontothe silica gel; this process is repeated until all the material has been loadedonto the silica gel. The ketone is eluted using 500 mL of1:1 ether:hexane and the eluent is concentrated by rotaryevaporation to afford the crude ketone as a white solid. This material isdissolved in 40-45 mL of boiling hexane. Upon cooling the solution to room temperature,the ketone begins to crystallize. The flask is then cooled to −25 °C for 2 hr. The resulting solids arecollected by filtration, washed with three 25-mL portionsof cold (−25 °C) hexane, and dried to afford 8.84-9.08 g, (86-88%)of the ketone as a white solid.
Notes：PCC is prepared by addition of pyridine to a solution of chromium trioxide (CrO3)in aqueous HCland crystallization.
Reference:OrganicSyntheses, Vol. 80, p.1
【WO2005037832 (2005)】
【Synlett2004, 738–740】
参考文献
一、Name Reactions （A Collection of Detailed Reaction Mechanisms）, Jie Jack Li, PCC oxidation，page 336.
二、common oxidation reagents, Yue Xu, Sundia Meditech
三、药名标准操作